As is well known, during the production of steel it is customary to condition the surfaces thereof at an intermediate stage of the steel making process. The purpose of such conditioning is to eliminate surface defects such as cracks, seams and slag inclusions that would produce defects in the finished products if not removed. It has been found economical to eliminate such defects from a workpiece by thermochemically removing the surface layer of one or more surfaces or portions thereof, by the use of a suitable machine, preferably interposed in the mill conveyor line between roll stands. In this way, the steel workpiece may be conditioned while hot and without interrupting continuous production of steel.
In many instances, the steel mill does not produce merely one size workpiece and often, many different sizes are produced in the same mill; it is common practice to change sizes between alternate workpieces in some mills. It has been the practice to remove such surface defects from workpieces such as billets, ingots, blooms, slabs and the like, by means of a scarfing machine having a single nozzle adapted to discharge a jet of oxygen along and acutely to an entire ferrous metal surface for the thermochemical reaction with surface metal, or by directing several oxygen streams from a plurality of nozzles disposed in a row or bank.
More recently, efforts have been made to employ an energy-saving, yield-increasing expedient known as hot charging, or direct rolling, whereby the semifinished steel product emerging from the continuous casting or slabbing process is immediately fed, while hot, to a working station. If seams, slag and/or other flaws in the steel surface were left unremoved in this stage, objectionable defects of a permanent nature would be developed in the finished product.
As indicated, these flaws usually have been removed by scarfing of the entire surface, but the practice involves the loss of significant amounts of metal and it is being replaced by localized scarfing for selective removal of the flaws (commonly referred to as selective, spot or band scarfing). Many different techniques and apparatus have been developed for selective or spot scarfing of metal surfaces to overcome the shortcomings indicated above. For example, in selective scarfing, generally several abutting individual scarfing nozzles are disposed transversely across the path of movement of the metal workpiece and are selectively operated so as to scarf only those areas containing surface defects, rather than the entire work surface.
Conventional scarfing processes have employed a wide variety of nozzles. The most common shapes of the oxygen discharge orifice have been either round (such as shown in U.S. Pat. No. 2,309,096 to Buckman et al.), slotted with round ends (such as shown in U.S. Pat. No. 2,664,368 to Babcock et al.), or a continuous slot (described in U.S. Pat. No. 2,622,048 to Moesinger and U.S. Pat. Nos. 2,838,431 and 3,231,431 to Allmang et al.). With such scarfing techniques, along the margins or boundaries of the treated area, slag and waste material, including molten metal, tend to flow from the undercut space and accumulate in a thin layer or film, a portion of which adheres to the metal surface, generally along the cut edges, and is termed a "fin". The slag is not all iron oxide but contains considerable metallic iron which has been displaced in the molten state due to the heat of reaction. Such "fin" formations are highly objectionable because they form surface defects when for example, rolled into the work by subsequent rolling operation; and it is most desirable that they be reduced to a minimum or, preferably, eliminated to prevent blemishes in the finished products.
In an effort to minimize the problem of "fin" formation, generally, it has been a practice to use the above types of nozzles in conjunction with jets of air, water or the like which are directed at the incipiently forming "fins" so as to push the "fin" forming molten metal away from the unscarfed work surface before it can solidify.
More recently, spot scarfing nozzles capable of individually scarfing randomly located defects in a metal body without forming "fins" along the boundaries of the scarfing cut have been disclosed, for example, in U.S. Pat. Nos. 4,013,486 and 4,040,871 to Engel which involve changing the geometry of the scarfing nozzle slot in a manner that reduces the amount of oxygen flow from the edges of the nozzle. Individual scarfing nozzles such as disclosed in U.S. Pat. No. 4,040,871, however, produce cuts which are narrower than the width of the nozzle discharge orifice. Thus, if two of these nozzles are aligned side-by-side to make two adjacent cuts in a single pass, an unscarfed or reduced scarfed area will remain between the cuts. Nozzles such as disclosed in U.S. Pat. No. 4,013,486 substantially eliminate the unscarfed or reduced scarfed area between the cuts of nozzles aligned side-by-side, but achieve that end only by changing the geometry of the nozzle slot, which reduces the flexibility thereof for use under varying scarfing conditions and for being automated.
One type of spot scarfing machine is commercially used to desurface the entire workpiece or, alternatively, to selectively scarf randomly located defects. It is composed of a plurality or bank of two or more adjacent scarfing nozzles, each of which is butted, side-by-side, with other like scarfing nozzles. Such machines generally include a starting method so that the scarfing reaction can be started at any point along the length of the steel workpiece. In such machines, it would be highly desirable to totally eliminate the problem of having unscarfed areas, or reduced scarfed areas, between adjacent cuts when a defect as wide or wider than the width of the scarfing nozzle is to be scarfed, as well as being capable of making a "fin" free selective scarfing cut with scarfing units which could be operated automatically and, preferably, could be remotely controlled.
It is known that among the many variables and conditions affecting and necessary to sustain the scarfing reaction (i.e., thermochemical, exothermic reaction for metal surface removal), the purity of the oxygen in the cutting fluid must be of a specific level. Thus, the oxygen purity of a scarfing nozzle oxygen stream provides a means for controlling the scarfing reaction to reduce the metal removal to zero. Heretofore, various ways have been suggested which generally apply this factor as one of the elements in producing a "fin" free scarfing pass including the use of a variety of auxiliary scarfing unit apparatus and nozzle configurations intended to dispense oxygen of reduced purity or non-oxygen containing gases at or near the edges of a scarfing nozzle.
Other suggested means for acquiring "fin" free scarfing typically involve the use of external jets, nozzles and the like to dispense a fluid, such as air or water, which is directed in such a way as to blow away molten steel at the pass edges. Thus, the force of fluid velocity and momentum are the principles used to eliminate the "fin."
Still other means of producing a "fin" free scarfing pass involve the principle of reducing oxygen flow. In a typical application such as disclosed in U.S. Pat. No. 4,013,486, the geometry of the scarfing nozzle slot is changed in a manner that reduces the normal nozzle opening, thus reducing the amount of oxygen flow. Other variations include a nozzle of normal geometry, but with reduced oxygen pressure. In either case, the flow of oxygen is reduced to a point where it is insufficient to sustain the scarfing reaction, resulting in scarfing terminating at the pass boundary with no "fin" being formed.
While many of the known processes and apparatus have been used commercially, they have been found to exhibit certain disadvantages such as: the complexity of the specially configured nozzles or jets; lack of flexibility in dealing with variables in steel compositions, scarfing speed, steel temperature, oxygen flow and pressure and the like; limitations in automatability of the process or the apparatus; and the like, and it would be highly desirable if a more flexible and, preferably, a readily automatable or remotely selectable system could be developed, particularly if such improvements resulted in less complex and costly apparatus and systems.